Method for producing closed-cell rigid polyurethane foams having low thermal conductivity

ABSTRACT

Closed cell rigid polyurethane foams are produced by reacting a) a polyol component with b) a polyisocyanate having an NCO content of from about 20 to about 48% by weight. The polyol component a) includes: 1) from 40 to 80% by weight, based on the total weight of polyol component a), of a sucrose or sorbitol initiated polyether having a molecular weight of from about 300 to about 800; 2) from 3 to 40% by weight, based on the total weight of polyol component a), of an aromatic amine initiated polyether having a molecular weight of from about 300 to about 800; 3) from 3 to 40% by weight, based on the total weight of polyol component a), of an aliphatic amine initiated polyether having a molecular weight of from about 200 to about 800; 4) from 3 to 40% by weight, based on the total weight of polyol component a), of a linear polyether having a molecular weight of from about 500 to about 1500; optionally, 5) other compounds containing at least isocyanate-reactive hydrogen atoms having a molecular weight of from about 150 to about 12,500; 6) a catalyst; 7) water; 8) a foaming agent; and optionally, 9) a processing aid and/or additive.

On account of their low thermal conductivity, polyurethane rigid foamedmaterials are employed for the insulation of refrigeration and freezingappliances, of industrial installations, of tank farms and pipelines,and in shipbuilding and in the construction industry. A summarisedreview of the production of polyurethane rigid foamed materials andtheir use is given in the Kunststoff-Handbuch, Volume 7 (Polyurethanes),2nd Edition 1983, edited by Dr. Günter Oertel (Carl Hanser Verlag,Munich).

The thermal conductivity of a substantially closed cell polyurethanerigid foamed material depends to a considerable extent on the type offoaming agent or cell gas used. Fully halogenated chlorofluorocarbons(CFCs) have proved to be particularly suitable for this purpose,particularly trichlorofluoromethane (R11), which has a particularly lowthermal conductivity. These substances are chemically inert andnontoxic. On account of their high stability, however, fully halogenatedchlorofluorocarbons enter the stratosphere, where they contribute to thedestruction of the ozone which is present there due to their chlorinecontent (Molina, Rowland, Nature 249 (1974) 810).

As a replacement for chlorofluorocarbons, the use of hydrofluoroalkanessuch as R141b (dichlorofluoroethane) as foaming agents has been proposed(U.S. Pat. No. 5,272,183).

The use of partially fluorinated hydrocarbons (hydrofluoroalkanes) asfoaming agents, which still contain at least one carbon-hydrogen bond,has also been proposed (EP 344 537, U.S. Pat. No. 4,931,482). Substancesof this class of compounds contain no chlorine atoms and consequentlyhave an ODP value (ozone depletion potential) of zero (by comparison,R11has an ODP of 1). Typical representatives of this class of substancesare, for example, 1,1,1,4,4,4-hexafluorobutane (R356) and1,1,1,3,3-pentafluoropropane (245fa).

In addition, the use of cyclopentane, or cyclopentane in admixture withother alkanes, such as i-butane, n-butane, n-pentane or isopentane, asfoaming agents is known from EP 0 421 269.

Depending on the foaming agent used, the polyol formulation has to bemodified in order to obtain polyurethane rigid foamed materials ofcomparable, low thermal conductivity. Polyol formulations wouldtherefore be desirable which would result in rigid foamed materials ofsimilar thermal conductivity when different foaming agent systems areused, without changes in the formulation or changes in the mechanicalequipment parameters for the foaming formulation becoming necessary,which are disadvantageous for reasons of cost.

The object of the present invention was therefore to identify a methodof producing closed cell polyurethane rigid foamed materials of lowthermal conductivity, with which polyurethane rigid foamed materialshaving a similar, low thermal conductivity can be obtained even whendifferent foaming agent systems are used.

Surprisingly, polyol formulations have now been found which can befoamed using all the aforementioned replacement substances forchlorofluorocarbons, particularly cyclopentane, cyclopentane/i-butanemixtures, cyclopentane/i-pentane mixtures, R 141b and R 245 fa, andwhich always result in polyurethane rigid foamed materials which have asimilar, low thermal conductivity and an outstanding spectrum ofproperties, and which are particularly suitable for the insulation ofrefrigeration and freezing appliances. This is particularly advantageousfor the processor of such formulations, since on a possible change offoaming agent, e.g. for legal reasons, no special adaptation ofmechanical equipment parameters and no change in stockholding isnecessary, and a change of foaming agent can thus be made rapidly andcosteffectively.

The present invention relates to a method of producing closed cellpolyurethane rigid foamed materials having a low thermal conductivityfrom polyols and polyisocyanates and from foaming agents and optionallyfrom foaming aids, characterised in that the polyurethane rigid foamedmaterial is obtained by the reaction of

A. a polyol component containing

(1) 40-80% by weight of a polyethylene oxide/propylene oxide polyetherof molecular weight 300-800 which is based on sucrose or sorbitol,

(2) 3-40% by weight of a polyethylene oxide/propylene oxide polyether ofmolecular weight 300-800 which is based on an aromatic mono-, di- orpolyamine,

(3) 3-40% by weight of a polyethylene oxide/propylene oxide polyether ofmolecular weight 200-800 which is based on an aliphatic mono-, di- orpolyamine,

(4) 3-40% by weight of a linear polyethylene oxide/propylene oxidepolyether of molecular weight 500-1500,

(5) optionally other compounds of molecular weight 150 to 12,500 g/molewhich comprise at least two hydrogen atoms which are reactive towardsisocyanates,

(6) catalysts,

(7) water,

(8) foaming agents from the group comprising alkanes, cycloalkanes,hydrochlorofluorocarbons, hydrofluorocarbons, and

(9) optionally adjuvant substances and/or additives with

B. a polyisocyanate which is optionally modified and which has an NCOcontent of 20 to 48% by weight.

Polyol formulations which are used according to the invention contain acompound of molecular weight 300 to 800 g/mole which comprises at leasttwo hydrogen atoms which are reactive towards isocyanates. They areobtained by the addition polymerisation of alkylene oxides, such asethylene oxide, propylene oxide, butylene oxide, dodecyl oxide orstyrene oxide for example, preferably propylene oxide or ethyleneoxides, with starter compounds. Polyhydric alcohols such as sucrose andsorbitol, as well as mixtures of these alcohols with water, glycerol,propylene glycol, ethylene glycol or diethylene glycol, are used asstarter compounds. By means of these polyols which are used according tothe invention, the mechanical properties of polyurethane rigid foamedmaterials which are usually required in practice can be obtained.

The polyol formulations which are used according to the inventionadditionally contain at least one compound which has a molecular weightof 300 to 800 g/mole and which comprises at least two hydrogen atomswhich are reactive towards isocyanates. These compounds are obtained bythe addition polymerisation of alkylene oxides, such as ethylene oxide,propylene oxide, butylene oxide, dodecyl oxide or styrene oxide forexample, preferably propylene oxide or ethylene oxide, with aromaticmono-, di- or polyamines such as aniline, phenylenediamine,diaminotoluenes (2,3-diaminotoluene, 3,4-diaminotoluene,2,4-diaminotoluene, 2,5-diaminotoluene, 2,6-diaminotoluene or mixturesof said isomers), 2,2′-diamino-diphenylmethane,2,4′-diamino-diphenylmethane, 4,4′-diaminodiphenylmethane or mixtures ofthese isomers.

In addition, the polyol formulations which are used according to theinvention contain at least one compound which has a molecular weight of200 to 800 g/mole and which comprises at least two hydrogen atoms whichare reactive towards isocyanates, which are obtained by the additionpolymerisation of alkylene oxides, such as ethylene oxide, propyleneoxide, butylene oxide, dodecyl oxide or styrene oxide for example,preferably propylene oxide or ethylene oxide, with aliphatic mono-, di-or polyamines such as ethylenediamine, oligomers of ethylenediamine (forexample diethylenetriamine, triethylenetetramine orpentaethylenehexamine), ethanolamine, diethanolamine, triethanolamine,N-methyl- or N-ethyldiethanolamine, 1,3-propylenediamine, 1,3- or1,4-butylenediamine, or 1,2-, 1,3-, 1,4-, 1,5- or1,6-hexamethylenediamine.

The polyol formulations which are used according to the invention alsocontain a compound which has a molecular weight of 500 to 1500 g/moleand which contains at least two hydrogen atoms which are reactivetowards isocyanates. These are obtained by the addition polymerisationof alkylene oxides, such as ethylene oxide, propylene oxide, butyleneoxide, dodecyl oxide or styrene oxide for example, preferably propyleneoxide or ethylene oxide, with starter compounds such as water, propyleneglycol, ethylene glycol or diethylene glycol.

The catalysts which are customary in polyurethane chemistry can be usedin the method according to the invention. Examples of catalysts of thistype include: triethylenediamine, N,N-dimethylcyclohexylamine,tetramethylenediamine, 1-methyl4-dimethyl-aminoethylpiperazine,triethylamine, tributylamine, dimethylbenzylamine,N,N′,N″-tris-(dimethylaminopropyl)-hexahydrotriazine,dimethylamino-propylformamide, N,N,N′,N′-tetramethylethylenediamine,N,N,N′, N′-tetramethylbutanediamine, tetramethylhexanediamine,pentamethyldiethylenetriamine, tetramethyldiaminoethyl ether,dimethylpiperazine, 1,2-dimethylimidazole, 1-aza-bicyclo-(3,3,0)-octane,bis-(dimethylaminopropyl)-urea, N-methylmorpholine, N-ethylmorpholine,N-cyclohexylmorpholine, 2,3-dimethyl-3,4,5,6,-tetrahydropyrimidine,triethanolamine, diethanolamine, triisopropanolamine,N-methyldiethanolamine, N-ethyldiethanolamine, dimethylethanolamine,tin(II) acetate, tin(II) octoate, tin(II) ethylhexoate, tin(II) laurate,dibutyltin diacetate, dibutyltin dilaurate, dibutyltin maleate,dioctyltin diacetate,tris-(N,N-dimethyl-aminopropyl)-s-hexahydrotriazine,tetramethylammnonium hydroxide, sodium acetate, potassium acetate,sodium hydroxide, or mixtures of these or similar catalysts.

The polyol formulations which are used in the method according to theinvention contain 0.5 to 7.0 parts by weight, preferably 1.0 to 3.0parts by weight, of water per 100 parts by weight of polyol component A.

Foaming agents used according to the invention are selected from thegroup comprising alkanes, particularly those comprising 4 to 5 C atoms,such as i-pentane, n-pentane, n-butane, isobutane, 2,2-dimethylbutane,or cycloalkanes, particularly cyclopentane or cyclohexane, orhydrochlorofluorocarbons, preferably R 141 b (dichlorofluoroethane) orhydrofluorocarbons, preferably R 245fa, as well as mixtures thereof.

Examples of the isocyanate component include aromatic polyisocyanates,such as those described by W. Siefken in Justus Liebigs Annalen derChemie, 562, pages 75 to 136, for example those of formula

Q(NCO)_(n),

wherein

n denotes 2 to 4, preferably 2, and

Q denotes an aliphatic hydrocarbon radical containing 2 to 18,preferably 6 to 10, C-atoms, a cycloaliphatic hydrocarbon radicalcontaining 4 to 15, preferably 5 to 10, C-atoms, or an aromatichydrocarbon radical containing 8 to 15, preferably 8 to 13, C-atoms,e.g. those polyisocyanates which are described in DE-OS 28 32 253, pages10 to 11.

Generally, polyisocyanates which are readily available commercially areparticularly preferred, e.g. toluene 2,4- and 2,6-diisocyanate, as wellas any mixtures of these isomers (“TDI”), polyphenylpolymethylenepolyisocyanates, such as those which are produced byaniline-formaldehyde condensation and subsequent phosgenation (“crudeMDI”), and “modified polyisocyanates” which contain carbodiimide groups,urethane groups, allophanate groups, isocyanurate groups, urea groups orbiuret groups. Modified polyisocyanates which are derived from toluene2,4- and 2,6-diisocyanate or from 4,4′- and/or 2,4′-diphenylmethanediisocyanate are particularly preferred.

Prepolymers of said isocyanates and organic compounds comprising atleast one hydroxyl group can also be used, for example, polyol orpolyester components which have a molecular weight of 60 to 1400 andwhich comprise 1 to 4 hydroxyl groups.

Paraffins or fatty alcohols or dimethylpolysiloxanes, as well aspigments or colorants, stabilisers against the effects of ageing andweathering, plasticisers, and substances having a fungistatic orbacteriostatic effect, can be used in conjunction, as can fillers suchas barium sulphate, silaceous earth, carbon black or whitening.

Other examples of surface-active additives and foam stabilisers, as wellas cell regulators, reaction retarding agents, stabilisers,flame-retardant substances, colorants and fillers, as well as substanceshaving a fungistatic and bacteriostatic effect, and details of themethod of use and mode of action of these additives, are described inthe Kunststoff-Handbuch, Volume VII, edited by Vieweg and Höchtlen, CarlHanser Verlag, Munich 1966, e.g. on pages 121 to 205, and in the 2ndEdition 1983, edited by G. Oertel (Carl Hanser Verlag, Munich).

For the production of foam, foaming is carried out according to theinvention in closed moulds. In the course of this procedure, thereaction mixture is introduced into a mould. Suitable mould materialsinclude metal, e.g. aluminium, or plastics, e.g. epoxy resin. Thereaction mixture foams in the mould and forms the moulding. Foaming inthe mould can be conducted so that the moulding has a cellular structureat its surface. However, it can also be conducted so that the mouldinghas a solid skin and a cellular core. In the first-mentioned case, theprocedure according to the invention consists of introducing sufficientfoamable reaction mixture into the mould so that the foamed materialwhich is formed just fills the mould. The procedure in the latter caseconsists of introducing more foamable reaction mixture into the mouldthan is necessary for filling the interior of the mould with foam. Inthe latter case, “over-charging” is thus employed. A procedure of thistype is known from U.S. Pat. No. 3,178,490 and U.S Pat. No. 3,182,104.

The method according to the invention is preferably used for foaming thehollow spaces of refrigeration and freezing appliances. Foamed materialscan also of course be produced by block foaming or by the doubleconveying process which is known in the art.

The rigid foamed materials which are obtainable according to theinvention are employed in the building trade and for the insulation oflong-run heating pipes and containers.

The present invention also relates to the use of the rigid foamedmaterials produced according to the invention as an intermediate layerfor composite elements and for the foaming of hollow spaces in thehousings of domestic refrigerators.

The following examples serve to explain the invention, without limitingthe scope thereof, however.

EXAMPLES

Raw Materials Used:

Polyol A: a polypropylene oxide/ethylene oxide polyether of molecularweight 550, based on sucrose/glycerol

Polyol B: a polypropylene oxide/ethylene oxide polyether of molecularweight 560, based on a mixture of 2,3- and 2,4-diaminotoluene

Polyol C: a polypropylene oxide polyether of molecular weight 540, basedon ethylenediamine

Polyol D: a polypropylene oxide polyether of molecular weight 900, basedon propylene glycol.

A mixture according to the invention was prepared from polyethers A-D ina ratio of 50:25:15:10 (w:w:w:w). This mixture was processed, inaccordance with the formulations given in the following Table, in a HK270 high-pressure machine supplied by Hennecke at 20° C. to formpolyurethane rigid foamed materials.

catalyst: dimethylcyclohexylamine (Bayer AG, Leverkusen)

silicone stabiliser: a commercially available silicone stabilisersupplied by Goldschmidt AG, Essen

cyclopentane: supplied by Erdolchemie, Dormagen

isobutane: supplied by Linde AG

isopentane: supplied by Exxon

R 141b: supplied by Solvay

R 245 fa: supplied by Allied Signal

isocyanate: “polymeric MDI” Desmodur® 44V20 (Bayer AG, Leverkusen).

Example Example Example Example Example 1 2 3 4 5 polyol mixture 100 100100 100 100 silicone stabiliser 2 2 2 2 2 catalyst c-pentane 14c-pentane/i-butane (80:20) 14 c-pentane/i-pentane (80:20) 14 R 141 b 31R 245 fa 28 isocyanate 142 142 142 142 142 surface apparent densitykg/m³) 22.0 21.6 21.0 21.8 24 core apparent density kg/m³) 32.6 31.531.0 31.4 32 thermal conductivity according to DIN 20.6 21.4 21.6 18.519.9 52616-77 (mW/m.K) compressive strength according to DIN 0.15 0.150.15 0.13 0.16 52421-84 (MPa) dimensional stability according to DIN 0 00 0 0 53431-77, 3 hours, −30° C.; change in dimensions in %

Examples 1 to 5 show that when the polyol formulations according to theinvention are foamed with commercially available foaming agents, rigidfoamed materials are always obtained which exhibit outstandingproperties which comply with what is required in practice. This isachieved with what is recognisable to one skilled in the art as a lowcore apparent density.

What is claimed is:
 1. A process for the production of a rigidpolyurethane foam comprising reacting a) a polyol componentcomprising 1) from about 40 to about 80% by weight, based on the totalweight of polyol component a), of a polyethylene oxide/propylene oxidepolyether based on sucrose or sorbitol having a number average molecularweight of from about 300 to about 800, 2) from about 3 to about 40% byweight, based on the total weight of polyol component a), of apolyethylene oxide/polypropylene oxide polyether based on an aromaticamine having a number average molecular weight of from about 300 toabout 800, 3) from about 3 to about 40% by weight, based on the totalweight of polyol component a), of a polyethylene oxide/propylene oxidepolyether based on an aliphatic amine having a number average molecularweight of from about 200 to about 800, 4) from about 3 to about 40% byweight, based on the total weight of polyol component a), of a linearpolyethylene oxide/propylene oxide polyether having a number averagemolecular weight of from about 500 to about 1500, optionally, 5) acompound containing at least two isocyanate-reactive hydrogen atomshaving a number average molecular weight of from about 150 to about12,500 which is not a polyether within 1), 2), 3) or 4), 6) a catalyst,7) water, 8) an alkane, cycloalkane, hydrochlorofluorocarbon orhydrofluorocarbon foaming agent, and optionally, 9) a processing aidand/or additive with b) a polyisocyanate or modified polyisocyanatehaving an NCO content of from about 20 to about 48% by weight.
 2. Theprocess of claim 1 in which polyether 1) is a sucrose initiatedpolyether having a 1,2-propylene oxide content of from 70 to 100% byweight and an ethylene oxide content of from 0 to 30% by weight.
 3. Theprocess of claim 1 in which polyether 2) is an o-toluene diamineinitiated polyether polyol having a 1,2-propylene oxide content of from70 to 100% by weight and an ethylene oxide content of from 0 to 30% byweight.
 4. The process of claim 1 in which polyether 3) is an ethylenediamine initiated polyether polyol having a 1,2-propylene oxide contentof from 50 to 100% by weight and an ethylene oxide content of from 0 to50% by weight.
 5. The process of claim 1 in which polyether 1) is asorbitol initiated polyether polyol having a 1,2-propylene oxide contentof from 70 to 100% by weight and an ethylene oxide content of from 0 to30% by weight.
 6. The process of claim 1 in which polyether 4) is apropylene glycol initiated polyether polyol having a 1,2-propylene oxidecontent of from 70 to 100% by weight and an ethylene oxide content offrom 0 to 30% by weight.
 7. The process of claim 1 in which water 7) ispresent in an amount of from 0.5 to 7.0% by weight for each 100 parts byweight of polyol component a).
 8. The process of claim 1 in which thefoaming agent 8) is cyclopentane and/or cyclohexane.
 9. The process ofclaim 1 in which the foaming agent 8) is a mixture composed of at leasttwo of the following foaming agents: n-pentane, isopentane, cyclopentaneand cyclohexane.
 10. The process of claim 1 in which the foaming agent8) is a mixture composed of at least two of the following foamingagents: cyclopentane, n-butane, isobutane, and 2,2-dimethylbutane. 11.The process of claim 1 in which the foaming agent 8) is n-pentane and/orisopentane.
 12. The process of claim 1 in which the foaming agent 8) is1,1,1-dichlorofluoroethane or 1,1,1,3,3-pentafluoropropane.